The invention relates to a water jet propulsion unit, comprising a jet tube, open at its upper and lower end, and accommodating therein a rotatingly driven propulsion means with in the outflow opening of the unit a rotatable grid of essentially mutually parallel guide plates crossing the axis of the propulsion element under an acute angle.
Such a waterjet propulsion unit is known in itself from the French patent specification 1478733; the propulsion means as used therein is a thrust propeller. This known unit comprises an outer drum around a circular opening in the bottom of the vessel, extending conically upwardly with therein a jet tube, converging downwardly, and with therebelow the grid guide plates.
Such a device is, at best, only suited for handling small powers. The inflow of the water at the upper end of the jet tube will be accompanied by heavy turbulences, resulting in a very low propeller efficiency and cavitation of the propeller. The plane of the exit grid essentially coincides with the plane of the bottom of the vessel so that a great amount of water will be pumped around: part of the outflowing water will be mixed with the inflowing water so that the efficiency of the propulsion unit will be still worse.
The invention aims to provide a propulsion unit as described hereinabove which, having a high efficiency, is suitable to handle high powers. According the invention this aim is obtained in that the jet tube is, outside the inlet end, surrounded by a curved guide surface which has, in cross-section in a radial plane through the axis of the jet tube and starting from the wall of said tube an at least semi-circular configuration, and ends at the outlet end in a guide chamber in which the end edges of two opposite walls, which bound the outflow opening of said chamber, cross the axis of the tube under an acute angle, said grid of plates being provided in said outflow opening with at least the end parts of the plates parallel to said end edges, the whole being rotatable, together with the grid, around the axis of the propulsion means.
The upper surround of the jet tube as proposed by the invention ensures a smooth inflow of the water therein, eliminating turbulences and propeller cavitation. The water which flows out of the jet tube does not directly hit the guide plates but flows firstly in the, converging, chamber in which the water is already given a direction which includes an angle with the axis of the propulsion means. By this the xe2x80x9cpumping aroundxe2x80x9d, which occurs in the unit according to the prior art, is prevented.
Preferably the propulsion means is a propulsion propeller with fixed or variable pitch. It is also possible to use a xe2x80x9cmixed flowxe2x80x9d propeller or a pumping disc.
In a preferred embodiment the chamber is fixed to the jet tube and rotatable, together therewith, around the axis of the propulsion means by means of drive means, acting on the jet tube.
Contrary to the prior art according to which the guide grid can be rotated by means of a submerged set of gears, in the proposed embodiment the rotating of the jet tube is effected by driving its support, thus in the dry zone within the vessel.
Contrary to the prior art in a preferred embodiment of the invention the jet tube is surrounded by an essentially circle-cylindrical drum of which the lower plane essentially coincides with the plane of guide plate grid. By this measure a better guiding of the water, which is sucked in around the jet tube, up to the upper end of the jet tube is obtained.
Furthermore preferably the guide plates comprise a first part lying near to the outflow opening of the chamber, parallel with the chamber valves which cross the axis of the propulsion means, and a second part, lying further within the chamber and crossing the said axis under a smaller angle.